Our transition from being Amish to, well, not being Amish has been interesting in many different ways. I still find it interesting to compare things that are considered proper and polite in one culture that would be considered offensive and rude in another.
One such thing is among the Amish when we invited someone to come to our house for a meal we expected them to come with nothing except a hearty appetite. To have asked what food to bring along or showing up with something would have been an insult to the hostess and her cooking abilities.
After we left the Amish and people asked me what they could bring I realized they were used to offering to bring something along out of being considerate and helpful, and I now find myself accepting those offers every once in a while and even, gasp, asking what I can bring when we get an invitation. It really does help when preparing all the extra food to not have to be responsible for everything, but I will still refrain from showing up at an Amish house carrying a salad or some kind of dessert.
Edited to add: If you want to visit the Amish and take some kind of gift along food or otherwise they won't be offended. The offer of bringing food is insulting only within the community.